Chernobyl Nuclear Accident

Introduction

The source document for this Digest states:

I. PHYSICAL CONSEQUENCES OF THE ACCIDENT

7. The accident at the Chernobyl nuclear power station occurred during a low-power engineering test of the Unit 4 reactor. Safety systems had been switched off, and improper, unstable operation of the reactor allowed an uncontrollable power surge to occur, resulting in successive steam explosions that severely damaged the reactor building and completely destroyed the reactor. An account of the accident and of the quantities of radionuclides released, to the extent that they could be known at the time, were presented by Soviet experts at the Post-Accident Review Meeting at Vienna in August 1986 [I2]. The information that has become available since 1986 will be summarized in this Chapter.

8. The radionuclide releases from the damaged reactor occurred mainly over a 10-day period, but with varying release rates. An initial high release rate on the first day was caused by mechanical discharge as a result of the explosions in the reactor. There followed a five-day period of declining releases associated with the hot air and fumes from the burning graphite core material. In the next few days, the release rate of radionuclides increased until day 10, when the releases dropped abruptly, thus ending the period of intense release. The radionuclides released in the accident deposited with greatest density in the regions surrounding the reactor in the European part of the former Soviet Union.

A. THE ACCIDENT

9. The Chernobyl reactor is of the type RBMK, which is an abbreviation of Russian terms meaning reactor of high output, multichannel type. It is a pressurized water reactor using light water as a coolant and graphite as a moderator. Detailed information about what is currently known about the accident and the accident sequence has been reported, notably in 1992 by the International Atomic Energy Agency (IAEA) [I7], in 1994 in a report of the Massachusetts Institute of Technology [S1], in 1995 by the Ukrainian Academy of Sciences [P4], and in 1991 1996 by the Kurchatov Institute [B24, C5, K20, K21, S22, V4]. A simplified description of the events leading to the accident and of the measures taken to control its consequences is provided in the following paragraphs. As is the case in an accident with unexpected and unknown events and outcomes, many questions remain to be satisfactorily resolved.

10. The events leading to the accident at the Chernobyl Unit 4 reactor at about 1.24 a.m. on 26 April 1986 resulted from efforts to conduct a test on an electric control system, which allows power to be provided in the event of a station blackout [I2]. Actions taken during this exercise resulted in a significant variation in the temperature and flow rate of the inlet water to the reactor core (beginning at about 1.03 a.m.). The unstable state of the reactor before the accident is due both to basic engineering deficiencies (large positive coefficient of reactivity under certain conditions) and to faulty actions of the operators (e.g., switching off the emergency safety systems of the reactor) [G26]. The relatively fast temperature changes resulting from the operators’ actions weakened the lower transition joints that link the zirconium fuel channels in the core to the steel pipes that carry the inlet cooling water [P4]. Other actions resulted in a rapid increase in the power level of the reactor [I7], which caused fuel fragmentation and the rapid transfer of heat from these fuel fragments to the coolant (between 1.23:43 and 1.23:49 a.m.). This generated a shock wave in the cooling water, which led to the failure of most of the lower transition joints. As a result of the failure of these transition joints, the pressurized cooling water in the primary system was released, and it immediately flashed into steam.

11. The steam explosion occurred at 1.23:49. It is surmised that the reactor core might have been lifted up by the explosion [P4], during which time all water left the reactor core. This resulted in an extremely rapid increase in reactivity, which led to vaporization of part of the fuel at the centre of some fuel assemblies and which was terminated by a large explosion attributable to rapid expansion of the fuel vapour disassembling the core. This explosion, which occurred at about 1.24 a.m., blew the core apart and destroyed most of the building. Fuel, core components, and structural items were blown from the reactor hall onto the roof of adjacent buildings and the ground around the reactor building. A major release of radioactive materials into the environment also occurred as a result of this explosion.

12. The core debris dispersed by the explosion started multiple (more than 30) fires on the roofs of the reactor building and the machine hall, which were covered with highly flammable tar. Some of those fires spread to the machine hall and, through cable tubes, to the vicinity of the Unit 3 reactor. A first group of 14 firemen arrived on the scene of the accident at 1.28 a.m. Reinforcements were brought in until about 4 a.m., when 250 firemen were available and 69 firemen participated in fire control activities. These activities were carried out at up to 70 m above the ground under harsh conditions of high radiation levels and dense smoke. By 2.10 a.m., the largest fires on the roof of the machine hall had been put out, while by 2.30 a.m. the largest fires on the roof of the reactor hall were under control. By about 4.50 a.m., most of the fires had been extinguished. These actions caused the deaths of five firefighters.

13. It is unclear whether fires were originating from the reactor cavity during the first 20 h after the explosion. However, there was considerable steam and water because of the actions of both the firefighters and the reactor plant personnel. Approximately 20 h after the explosion, at 9.41 p.m., a large fire started as the material in the reactor became hot enough to ignite combustible gases released from the disrupted core, e.g. hydrogen from zirconium-water reactions and carbon monoxide from the reaction of hot graphite with steam. The fire made noise when it started (some witnesses called it an explosion) and burned with a large flame that initially reached at least 50 m above the top of the destroyed reactor hall [P4].

14. The first measures taken to control the fire and the radionuclide releases consisted of dumping neutron-absorbing compounds and fire-control materials into the crater formed by the destruction of the reactor. The total amount of materials dumped on the reactor was approximately 5,000 t, including about 40 t of boron compounds, 2,400 t of lead, 1,800 t of sand and clay, and 600 t of dolomite, as well as sodium phosphate and polymer liquids [B4]. About 150 t of materials were dumped on 27 April, followed by 300 t on 28 April, 750 t on 29 April, 1,500 t on 30 April, 1,900 t on 1 May, and 400 t on 2 May. About 1,800 helicopter flights were carried out to dump materials onto the reactor. During the first flights, the helicopters remained stationary over the reactor while dumping the materials. However, as the dose rates received by the helicopter pilots during this procedure were judged to be too high, it was decided that the materials should be dumped while the helicopters travelled over the reactor. This procedure, which had a poor accuracy, caused additional destruction of the standing structures and spread the contamination. In fact, much of the material delivered by the helicopters was dumped on the roof of the reactor hall, where a glowing fire was observed, because the reactor core was partially obstructed by the upper biological shield, broken piping, and other debris, and rising smoke made it difficult to see and identify the core location (see Figure I). The material dumping campaign was stopped on day 7 (2 May) through day 10 (5 May) after the accident because of fears that the building support structures could be compromised. If that happened, it would allow the core to be less restrained from possible meltdown, and steam explosions would occur if the core were to interact with the pressure suppression pool beneath the reactor. The increasing release rates on days 7 through 10 were associated with the rising temperature of the fuel in the core. Cooling of the reactor structure with liquid nitrogen using pipelines originating from Unit 3 was initiated only at late stages after the accident. The abrupt ending of the releases was said to occur upon extinguishing the fire and through transformation of the fission products into more chemically stable compounds [I2].

15. The further sequence of events is still somewhat speculative, but the following description conforms with the observations of residual damage to the reactor [S1, S18]. It is suggested that the melted core materials (also called fuel- containing masses, corium, or lava) settled to the bottom of the core shaft, with the fuel forming a metallic layer below the graphite. The graphite layer had a filtering effect on the release of volatile compounds. This is evidenced by a con- centration of caesium in the corium of 35% [S1], somewhat higher than would otherwise have been expected in the highly oxidizing conditions that prevailed in the presence of burning graphite. The very high temperatures in the core shaft would have suppressed plate-out of radionuclides and maintained high release rates of penetrating gases and aerosols. After about 6.5 days, the upper graphite layer would have burned off. This is evidenced by the absence of carbon or carbon- containing compounds in the corium. At this stage, without the filtering effect of an upper graphite layer, the release of volatile fission products from the fuel may have increased, although non-volatile fission products and actinides would have been inhibited because of reduced particulate emission.

16. On day 8 after the accident, it would appear that the corium melted through the lower biological shield (LBS) and flowed onto the floor of the sub-reactor region (see Figure I). This rapid redistribution of the corium and increase in surface area as it spread horizontally would have enhanced the radionuclide releases. The corium produced steam on contact with the water remaining in the pressure suppression pool, causing an increase in aerosols. This may account for the peak releases of radionuclides seen at the last stage of the active period.

17. Approximately nine days after the accident, the corium began to lose its ability to interact with the surrounding materials. It solidified relatively rapidly, causing little damage to metallic piping in the lower regions of the reactor building. The chemistry of the corium was altered by the large mass of the lower biological shield taken up into the molten corium (about 400 of the 1,200-t shield of stainless steel construction and serpentine filler material). The decay heat was significantly lowered, and the radionuclide releases dropped by two to three orders of magnitude. Visual evidence of the disposition of the corium supports this sequence of events.

18. On the basis of an extensive series of measurements in 1987 1990 of heat flux and radiation intensities and from an analysis of photographs, an approximate mass balance of the reactor fuel distribution was established (data reported by Borovoi and Sich [B16, S1]). The amount of fuel in the lower regions of the reactor building was estimated to be 135± 27 t, which is 71% of the core load at the time of the accident (190.3 t). The remainder of the fuel was accounted for as follows: fuel in the upper levels of the reactor building (38± 5 t); fuel released beyond the reactor building (6.7± 1 t); and unaccounted for fuel (10.7 t), possibly largely on the roof of the reactor hall under the pile of materials dumped by the helicopters.

19. Different estimates of the reactor fuel distribution have been proposed by others. Purvis [P4] indicated that the amount of fuel in the lava, plus fragments of the reactor core under the level of the bottom of the reactor, is between 27 and 100 t and that the total amount of the fuel in the reactor hall area is between 77 and 140 t. Kisselev et al. [K12, K15] reported that only 24± 4 t were identified by visual means in the lower region of the reactor. It may be that most of the fuel is on the roof of the reactor hall and is covered by the material that was dropped on it from helicopters. Only the removal of this layer of material will allow making a better determination of the reactor fuel distribution.

Source & ©: UNSCEAR 2000 Report, Volume II, Annex J,
"Exposures and effects of the Chernobyl accident"
 Chapter I: Physical consequences of the accident, Section A: The Accident